Prostate cancer remains the most common male malignancy and the second most common cause of cancer-related mortality in most Western societies. The incidence of prostate cancer and corresponding mortality rates vary strikingly among ethnic, racial, and national groups with noteworthy high rates among African Americans. The identification of an upregulated and strongly expressed antigenic marker on prostate cancer cells, namely prostate specific membrane antigen (PSMA), has attracted a great deal of attention as a target for immunotherapy. More recently, it has been reported that PSMA possesses at least two specific enzymatic activities; the hydrolytic cleavage and liberation of glutamic acid from both gamma-glutamyl derivatives of folic acid and the neuropeptide NAAG (Nacetylaspartylglutamate). Although these enzymatic activities have been clearly identified, questions of medical interest remain to be answered for PSMA. Two such questions are: "What is the role of PSMA on the surface of prostate cancer cells?" and "How would inhibiting PSMA affect the growth, proliferation, or regulation of prostate cancer cells?" Although the proposed research will not immediately address these questions, it is anticipated that the results of this work will provide the investigators with suitable tools to study these issues in future studies. Furthermore, it is likely that the enzymatic activity of PSMA could be exploited for chemotherapeutic strategies, one of which being the inhibition of its activity by small molecule inhibitors. The overall objective of the proposed work is to develop potent and selective inhibitors for PSMA. This will be accomplished by first conducting substrate studies to identify optimal structural frameworks for enzyme recognition. These frameworks will then be utilized in the design and development of a first generation of phosphonopeptide inhibitors of PSMA. The most promising and potent of these inhibitors will be then further elaborated with binding probes to identify and exploit auxiliary binding sites on the enzyme leading to enhanced inhibitory potency.